gitlab-ci: Move tags and image properties to a default section.

This avoids redundancy and conforms to more recent style.
3 jobs for master in 45 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Build
passed autotools #11954
docker

00:00:45

passed doc #11956
docker

00:00:14

passed makefile #11955
docker

00:00:12